This invention relates to inspection apparatus, and more particularly to metrological apparatus having a reticle projection system for determining precise dimensional measurements of manufactured objects. Even more particularly, this invention relates to variable magnification apparatus for providing a variable range of magnifications for a reticle projection system of the type described.
There are currently available in the marketplace a number of sophisticated metrological optical systems employed for purposes of quality control, and to determine the precise dimension of manufactured objects. Typically the machines incorporating such systems employ a magnification lens system for projecting the image of an object that is being inspected to a video camera for computerized detection of Features of the object in, for example, a horizontal plane--i.e. the XY plane. In addition, such machines also normally incorporate so-called autofocusing equipment which is utilized to determine the vertical distance (Z) to a spot on the surface of the object that is being inspected. The determination of the range or Z distance from a reference point to a spot on the object that is being inspected, can be computed by monitoring the sharpness of the image to an object that is being scanned through the best focus. This method of determining the Z distance depends upon the computational method used, the optical characteristics of the imaging system, the method of illumination of the object that is being inspected, and the characteristics of the object surfaces and Features.
Among the problems encountered in autofocusing methods of the type described, is that in those cases where the inspected object has a particularly smooth or transparent surface, the observed image of the surface has very little texture or contrast, which therefore makes it extremely difficult accurately to measure the Z distance to a particular point on the surface. In such cases it had been customary to project an image of a reticle or grid pattern to the object plane of the image, and the image of the pattern can then be observed when a real object surface is in position to reflect that pattern or grid. One such method of autofocusing is disclosed in U.S. Pat. No. 4,920,273, and is employed in equipment manufactured by Leitz Mestechnik GmbH, and identified as the Leitz UMS 432, Measuring Station.
The method employed in the two above-noted prior art devices comprises a method of artificial texture projection, which gives more precise results when the depth of focus of the projected grid or pattern is very shallow; and this depth of focus in turn depends upon the F-number of the reticle projector that is employed. More precise results are recognized also when the texture pattern is matched to the magnification employed to observe the object, and matches the focus analysis method employed. The machines disclosed in the above-noted U.S. patents however, do not employ any apparatus which assures the presence of these features in their equipment. Specifically, such prior art apparatus does not employ any means for separately providing the reticle projector with a range of magnifications which can be matched to the various object magnifications which normally are present in metrological apparatus of the type described.
It is an object of this invention, therefore, to provide for a reticle projection system in metrological apparatus of the type described, apparatus for effecting variable magnification of the projected reticle image.
Another object of this invention is to provide for a reticle projection system of the type described, apparatus which permits lower F-number and therefore shallower depth of focus of the projected image of a reticle or grid pattern.
A more specific object of this invention is to provide for a reticle projection system zoom magnification apparatus which is coupled to a video camera for movement therewith toward and away from an object plane to achieve synchronized magnification between the video camera and reticle projector.
Other objects of this invention will be apparent hereinafter from the specification and from the recital of the appended claims, particularly when read in conjunction with the accompanying drawings.